The present disclosure generally relates to an inductor including a substrate and a spiral inductor formed on the substrate, and to a method for fabricating the inductor.
With recent widespread use of cellular phones and personal digital assistants (PDAs), there has been an increasing demand for reducing the size of high-frequency circuits with wireless interfaces. To meet with this demand, passive components, such as inductors, which used to be provided outside semiconductor devices, e.g., mounted on printed wiring boards (i.e., passive components which used to be external components) are housed in the semiconductor devices in many cases. Inductors are components necessary for application to the design of radio frequency integrated circuits (RFICs) such as low noise amplifiers (LNAs), power amplifiers (PAs), radio frequency (RF) oscillators. However, as compared to resistors and capacitors that are relatively easily formed within chips, inductors have various structures for acquiring inductive properties, and are not necessarily easily formed within chips.
A conventional technique (e.g., a technique disclosed in Japanese Laid-Open Patent Publication No. 2002-134319) is now described with reference to FIGS. 8A and 8B. FIG. 8A is a plan view illustrating an inductor. FIG. 8B is a cross-sectional view taken along line VIIIB-VIIIB′ in FIG. 8A.
As illustrated in FIG. 8A, an inductor 100 has a structure where a conductor 101 is in a spiral shape. The outer and inner ends of the conductor 101 serve as a terminal 103a and a terminal 103b respectively. Current F which has entered the terminal 103a flows in the direction indicated by arrows F1, F2, F3, and F4, in this order, and is obtained from the terminal 103b. 
As illustrated in FIG. 8B, the inductor 100 (having turns 101a, 101b, and 101c) is provided in an interlayer insulating film 104 formed on a substrate. The conductor 101 is rectangular in a cross sectional taken along the width of the conductor 101.
To describe characteristics of an inductor, a quality factor (Q factor) is generally used, and is expressed as:Q=ωL/R  (1)where ω is 2πf, π is a circle ratio, f is a frequency, L is an inductance, and R is a resistance.
It is considered that as the Q factor increases, electric characteristics of the inductor are enhanced. A large Q factor contributes to reduction of power consumption of a circuit.
It is well known that the series resistance of a spiral inductor is a main cause of a decrease in the Q factor of the inductor. One of measures for reducing the series resistance of the inductor is using a wide conductor. However, an increase in the width of the conductor increases the area of the inductor, thereby disadvantageously increasing parasitic capacitance relating to the structure. Consequently, the self-resonant frequency of the inductor decreases, thus limiting the effective frequency of the inductor.
In the inductor 100 illustrated in FIG. 8A, the turns 101a through 101c of the conductor 101 are equally spaced from each other, and the inner turn 101c is narrower than the outer turn 101a. Accordingly, the distance from a center 105 to the inner turn 101c is larger than that in a configuration in which the inner turn 101c and the outer turn 101a have the same width. Consequently, each of opposing turns of the conductor 101 is not greatly affected by the line of magnetic force of the other turn. As a result, the inductance L increases, thus causing an increase in the Q factor.
FIG. 9 shows a Q characteristic (i.e., a variation of the Q factor with respect to the frequency) of a general inductor. As shown in FIG. 9, as the frequency increases, the Q factor temporarily increases. However, in a high-frequency range, the Q factor decreases because of a capacitive loss due to capacitive coupling between the inductor and the substrate. As a result, the Q characteristic has a convex shape which curves upward.
According to Equation (1), to increase the Q factor, reduction of the series resistance of the inductor and suppression of the capacitive loss are effective.